In recent years, mechanical drive systems in vehicles, are gradually being replaced by electrical drive-by-wire systems, in which the components are driven by electric motors for improved space efficiency, assemblability, controllability, and the like. One example of such replacement may be found in transmission shift position switching mechanisms.
In such a system, for example, as disclosed in a patent document 1 (i.e., Japanese patent No. 3886042), an encoder that is synchronized with a rotation of the motor outputs a pulse signal at every given angle of the motor, and an output value of the encoder is used to detect a rotation position of the motor.
In the patent document 1, the power supply phase of the motor is serially switched to rotate the motor to a target rotation position corresponding to a target shift position based on the encoder count (i.e., information of the rotation position of the motor), which is achieved by a feedback control of the rotation drive of the motor.
Further, during the feedback control, when an interval of two A phase signals or an interval of two B phase signals increases to be equal to or greater than an abnormality determination value, or when a difference between an A phase encoder count and a B phase encoder count increases to be equal to or greater than an abnormality determination value, it is determined that an abnormality is caused in the feedback control of the system, and the system switches the feedback control to an open-loop control.
However, in patent document 1, a one-time or temporary noise may be erroneously recognized as an output signal from the encoder. Further, a matching between the encoder count, the rotation position of the motor, and the power supply phase may be broken, which may result in a failure in the motor rotation control, in which a rotation drive of the motor is performed by serially switching the power supply phase based on the encoder count.